1. Field of the Invention
The present invention relates to an ink jet recording head, which is used in a recording apparatus for discharging recording liquid such as ink from a discharge port to form liquid droplets and perform recording operation, and a method of manufacturing the same. Incidentally, the ink jet recording head of the present invention can be applied to an apparatus such as a copying machine, a facsimile machine having a communication system and a wordprocessor having a printing unit in addition to a general printing apparatus, and further to an industrial recording apparatus that is compositely combined with various processing apparatuses.
2. Related Background Art
An ink jet recording apparatus is a recording apparatus of a so-called non-impact recording system and has a characteristic that it generates little noise at the time of printing and is capable of performing high-speed recording and recording on various recording media. Thus, the ink jet recording apparatus is widely employed as an apparatus for bearing a recording mechanism for a printer, a copying machine, a facsimile machine, a wordprocessor and the like.
As a representative ink discharge method in a recording head that is mounted in such an ink jet recording apparatus, there are known a method using an electromechanical transducing body such as a piezoelectric element, a method of irradiating an electromagnetic wave such as laser to cause ink to heat and discharging ink droplets by an action of the heating, a method of heating ink by an electrothermal conversion element having a heating resistor and discharging ink droplets by an action of film boiling, or the like.
Among these methods, the ink jet recording head using an electrothermal conversion element has an electrothermal conversion element provided in a recording liquid chamber, supplies an electric pulse being a recording signal to the element to cause it to heat, thereby giving thermal energy to ink, and utilizes a bubble pressure at the time of bubbling (boiling) of recording liquid caused then by phase change of the recording liquid to discharge ink liquid from a micro discharge port and record an image on a medium to be recorded. The ink jet recording head using an electrothermal conversion element generally includes a nozzle in which a discharge port for discharging ink droplets is opened, and an ink flow path and a common liquid chamber for supplying ink to this nozzle.
Such an ink jet recording head is usually mounted on a carriage of a recording apparatus main body. The recording apparatus main body includes conveying means for conveying a medium to be recorded such that it passes a position opposing a discharge port surface of the ink jet recording head mounted on the carriage. The carriage is configured to be movable in a direction perpendicular to a direction of conveying a medium to be recorded.
A recording operation in such a recording apparatus is performed by repeating main scanning for discharging ink at a predetermined period while moving the ink jet recording head and sub-scanning for conveying a medium to be recorded by a predetermined length.
FIGS. 45A and 45B are schematic views showing a nozzle part of a conventional ink jet recording head. FIG. 45A is a plan view showing a discharge port forming member in a transparent state and FIG. 45B is a sectional view cut along the line 45Bxe2x80x9445B of FIG. 45A. Reference symbol G denotes a central line of an ink flow path.
The ink jet recording head shown in FIGS. 45A and 45B includes a common liquid chamber 154 connected to an ink supply port 156. On both sides of the common liquid chamber 154, a plurality of electrothermal conversion elements 151 for causing ink to bubble to discharge the ink and a plurality of circular pressure chambers 155 having centers in common with the electrothermal conversion elements 151 are provided side by side. An ink flow path 153 is provided between the common liquid chamber 154 and each pressure chamber 155. A discharge port 152 is opened in a position opposing each electrothermal conversion element 151.
In this ink jet recording head, positions in a printing direction (carriage moving direction) of sets of the discharge port 152 and the electrothermal conversion element 151 that are adjacent to each other are shifted from one another by an offset equivalent to a distance that a carriage (not shown) moves during a lagged time of driving timing between each driving block. For simplicity of illustration, in FIGS. 45A and 45B, an ink jet recording head in which four driving blocks are allocated to each nozzle is shown, and an arrangement of the discharge port 152 in a printing direction periodically changes for every four nozzles in a direction of a row of discharge openings.
Then, if numbers are given to the driving blocks in the ascending order from the one to be driven first, in the example shown in FIGS. 45A and 45B, a driving block 1 is allocated to the discharge port 152 at the upper right and the discharge port 152 apart from it by the number of nozzles of integer times of four, a driving block 2 is allocated to the discharge ports 152 on the left of them, a driving block 3 is allocated to the discharge ports 152 on the left of the driving block 2, and a driving block 4 is allocated to the discharge ports 152 on the left of the driving block 3. With such a configuration, the driving blocks 1 to 4 are sequentially driven in the ascending order, whereby it becomes possible to discharge ink and cause the ink discharged from these discharge ports 152 to be applied on a recording medium in one row.
In a nozzle of the configuration shown in FIGS. 45A and 45B, since a central line of an ink flow path 163 and a central line of the electrothermal conversion element 151 coincide with each other, a flow of ink heading to the pressure chamber 155 from the common liquid chamber 154 through the ink flow path 163 is generated in line symmetry with respect to the central line of the electrothermal conversion element 151. Thus, bubbles generated by heating the ink by the electrothermal conversion element 151 disappear steadily on the electrothermal conversion element 151 in symmetry with respect to its central line. Although bubble disappearance positions are dispersed to corners (four corners in total) of a heating area of the electrothermal conversion element 151 in some cases, each bubble disappearance position is fixed even in such cases.
When the bubbles disappear, an impact force due to collapse of cavitation is generated. In the nozzle structure in which bubble disappearance positions are stable as in the above-mentioned conventional art, since a specific part of the electrothermal conversion element 151 is subject to an impact force due to the collapse of cavitation, the electrothermal conversion element 151 is susceptible to damages and hense its durable life is shortened.
The present invention has been devised in view of the above-mentioned drawbacks of the prior art, and it is an object of the present invention to provide an ink jet recording head that is capable of avoiding damages due to cavitation of an electrothermal conversion element and thus extending its life.
In order to attain the above-mentioned object, an ink jet recording head according to the present invention is an ink jet recording head comprising: a plurality of ink discharge ports for discharging ink; a plurality of electrothermal conversion elements that are provided to be associated with each of the ink discharge ports, respectively, for bubbling and discharging the ink; a plurality of pressure chambers for containing the electrothermal conversion elements and providing spaces for heating and bubbling the ink; a common liquid chamber for supplying ink to the plurality of pressure chambers; and a plurality of ink flow paths for communicating the pressure chambers with the common liquid chamber, which is characterized in that the ink flow paths are arranged such that central lines in a direction of ink supply to the pressure chambers are offset from central lines of the electrothermal conversion elements in the same direction.
According to this configuration, when bubbles for discharging ink are caused to disappear, the bubbles are washed to a position deviating to sides of the electrothermal conversion element by a flow of the ink refill upon the bubble disappearance. Thus, final bubble disappearance can be performed in this position and an adverse influence on the electrothermal conversion element due to cavitation at the time of bubble disappearance can be reduced.
In particular, in an ink jet recording head having pressure chambers of a substantially cylindrical shape, an ink flow path is arranged in a position offset from a central line of an electrothermal conversion element, whereby final bubble disappearance can take place in a relatively wide area extending vertically in the vicinity of side edges of the pressure chamber to thereby disperse areas of cavitation generation to reduce the influence of cavitation.
Moreover, an ink discharge port is arranged such that its center is positioned offset from the center of the electrothermal conversion element, whereby a direction of a velocity vector at the time when ink, which remains between the discharge port and a bubble after the bubbling and discharging an ink droplet from the discharge port (hereinafter referred to as ink on the discharge port side), moves toward the electrothermal conversion element following contraction of a bubble at the time of bubble disappearance can be fluctuated unstably or the velocity vector may be slanted with respect to the electrothermal conversion element rather than being perpendicular thereto. Moreover, it becomes possible to cover a portion on which the ink on the discharge port side collides against the electrothermal conversion element by ink flowing in from the common liquid chamber side (hereinafter referred to as ink on the liquid chamber side) before the ink on the discharge port side collides against the electrothermal conversion element.
As a result, the bubble disappearance process ends without the ink on the discharge port side vertically colliding against a part of the electrothermal conversion element intensively. Therefore, the electrothermal conversion element is not subject to a strong impact force in the bubble disappearance process and is hardly susceptible to damages. As a result, it becomes possible to remarkably improve durability performance of the electrothermal conversion element.
In addition, the ink jet recording head may have a configuration in which the center of the ink discharge port is arranged at a position offset to the ink flow path side from the center of the electrothermal conversion element. Thus, a direction of a velocity vector at the time when the ink on the discharge port side moves toward the electrothermal conversion element following contraction of a bubble at the time of bubble disappearance can be fluctuated unstably or the velocity vector may be made to be slanted with respect to the electrothermal conversion element rather than being perpendicular thereto. Moreover, it becomes possible to cover a portion on which the ink on the discharge port side collides against the electrothermal conversion element by the ink on the liquid chamber side flowing in from the common liquid chamber side before the ink on the discharge port side collides against the electrothermal conversion element.
Furthermore, it is preferable that the ink jet recording head has a configuration in which an amount of offset in the ink discharge port is 1 to 10 xcexcm. More preferably, the amount of offset is 3 to 7 xcexcm.
In addition, the ink jet recording head may have a configuration in which the center of the electrothermal conversion element is arranged to be positioned offset from the center of the pressure chamber. Thus, it becomes possible to set an offset amount between the center of the discharge port and the center of the electrothermal conversion element large while holding an offset amount of the center of the discharge port form the center of the pressure chamber small. As a result, a discharge direction of ink liquid droplets is maintained appropriately and a bubble collection generated in the pressure chamber is suppressed, whereby it becomes possible to prevent an ink accumulation from being formed on an outside surface in the vicinity of the discharge port and to keep a grade of a recorded image high.
In the ink jet recording head of the present invention, a bubble tends to be driven to the outside of an edge of a part of the ink discharge port communicating to the pressure chamber in the bubble disappearance process. Thus, it is also preferable that the ink jet recording head has a configuration in which an area occupied by the electrothermal conversion element is included in an area surrounded by the edge of the part of the ink discharge port communicating to the pressure chamber when it is viewed on a plane parallel with the surface of the pressure chamber to which the ink discharge port communicates. That is, with such a configuration, a bubble disappearance can occur in an area outside the electrothermal conversion element more surely and the influence of cavitation on the electrothermal conversion element can be further reduced.
In the case of this configuration, it is preferable to provide a taper on the side surface of the ink discharge port such that the cross section area increases toward the pressure chamber side. In this way, the area occupied by the electrothermal conversion element can be included in the area surrounded by the edge of the part of the ink discharge port communicating to the pressure chamber while holding a size of an opening on an ink discharge surface of the ink discharge port small as desired.
Moreover, if the ink discharge port has a taper as described above, it is preferable that a distance from the edge of the opening on the ink discharge surface side of the ink discharge port to the edge of the electrothermal conversion element is substantially equal at an arbitrary position in a part where the area occupied by the electrothermal conversion element goes over the edge of the opening on the ink discharge surface side of the ink discharge port when it is viewed on a plane parallel to a surface of the pressure chamber to which the ink discharge port communicates. In this way, a taper angle can be minimized.
In addition, if the center of the ink discharge port is arranged to be positioned offset from the center of the electrothermal conversion element, the ink discharge port preferably has a shape long in the direction offset from the electrothermal conversion element. In this case, the ink discharge port may be any of rectangular, ellipse or oval shape. In this way, the area occupied by the electrothermal conversion element can be included in the area surrounded by the edge of the part of the ink discharge port communicating to the pressure chamber while holding the size of the ink discharge port or its taper angle minimum.
In addition, the ink discharge port preferably has a shape long in the direction in which wiring for supplying electric power to the electrothermal conversion element is connected. In this case, the ink discharge port may be any of rectangular, ellipse or oval shape. According to this configuration, a connection part of the electrothermal conversion element and the wiring can be included in the area surrounded by the edge of the part of the ink discharge port communicating to the pressure chamber. Therefore, the influence of cavitation on the connection part can be reduced.
In addition, it is preferable that the ink jet recording head has a configuration in which the offset direction of the ink flow path from the central line of the electrothermal conversion element is the same for the plurality of ink flow paths arranged in one row. With this configuration, even if a position of formation of a member forming the ink flow path and the pressure chamber deviates from its original position due to production variance, a relative position of the ink flow path with respect to the electrothermal conversion element and the discharge port deviates similarly for any of a plurality of nozzles, whereby it becomes possible to make deviation not to occur in the ink discharge amount or the ink discharge direction among the plurality of nozzles and to make adverse influence on a formed image not to occur so frequently.
Similarly, it is preferable that the ink jet recording head has a configuration in which the ink flow path is formed in two rows side by side, opposingly on both sides of the common liquid chamber and the offset direction of the ink flow path belonging to the opposing ink flow path rows from the central line of the electrothermal conversion element is line symmetry with respect to a line parallel with a row direction of the opposing ink flow path rows.
In addition, in the ink jet recording head of the present invention, a flow resistance is made substantially equal in the plurality of ink flow paths with different lengths, whereby a refill property of the plurality of ink flow paths can be made substantially the same.
It is desirable to keep a difference of the flow resistances in the plurality of ink flow paths within 10% such that a satisfactory image with substantially no unevenness of density can be formed by making the refill property of the plurality of ink flow paths substantially the same and making a discharge amount of ink from the plurality of nozzles substantially equal at the time when ink is continuously discharged at a predetermined frequency.
The flow resistance of the plurality of ink flow paths with different lengths can be made substantially equal as described above by varying cross section areas of the plurality of ink flow paths with different lengths. In order to change the cross section areas of the ink flow paths, it is sufficient to change widths or heights of the ink flow paths or provide a rib in at least any one of the plurality of ink flow paths.
In the ink jet recording head of the present invention, if an area, in which a flow resistance per a unit length is smaller than the flow resistance of an area in the discharge port side of the ink flow path, is provided in an area on the common liquid chamber side of the ink flow path, even if a width of the common liquid chamber or the like deviates from an original width due to production variance, it is possible to make the flow resistances of the plurality of ink flow paths substantially equal. That is, since the flow resistance of the entire ink flow path is a sum of the flow resistance of each part, the flow resistance of the ink flow path is generally determined by the flow resistance of an area on the discharge port side where the flow resistance is relatively large. Thus, even if a length of the ink flow path of the common liquid chamber having a relatively small flow resistance changes a little, the flow resistance of the entire ink flow path hardly changes.
The above-mentioned ink jet recording head with different lengths of the plurality of ink flow paths, in particular, allocates an electrothermal conversion element to a plurality of driving blocks and drives the electrothermal conversion element at timing staggered for each driving block. Thus, the ink jet recording head is typically used as an ink jet recording head in which the plurality of ink discharge ports are arranged offset in a printing direction, and the present invention can be preferably applicable to such an ink jet recording head.
A method of manufacturing an ink jet recording head according to the present invention is characterized by having a step for finding a flow resistance R of an ink flow path by expressions shown below and determining a shape of the ink flow path such that the flow resistances are equal in the plurality of ink flow paths based on the obtained flow resistance;       R    =          η      ⁢                        ∫          0          L                ⁢                                            D              ⁢                              (                x                )                                                                    S                ⁡                                  (                  x                  )                                            2                                ⁢                      xe2x80x83                    ⁢                      ⅆ            x                                          D      ⁢              (        x        )              =          12.0      xc3x97              (                  0.33          +                      1.02            xc3x97                          (                                                                    a                    ⁢                                          (                      x                      )                                                                            b                    ⁢                                          (                      x                      )                                                                      +                                                      b                    ⁢                                          (                      x                      )                                                                            a                    ⁢                                          (                      x                      )                                                                                  )                                      )            
where,
x is a distance from the common liquid chamber;
S(x) is a cross section area of the ink flow path in a position of the distance x;
D(x) is a cross section coefficient of the ink flow path in the position of the distance x;
a(x) is a height of the ink flow path in the position of the distance x;
b(x) is a width of the ink flow path in the position of the distance x; and
xcex7 is an ink viscosity.
In addition, the method of manufacturing the ink jet recording head in accordance with the present invention may find the flow resistance R of the ink flow path by expressions shown below:       R    =          η      ⁢                        ∑                      n            =            1                    k                ⁢                  xe2x80x83                ⁢                                            D              ⁢                              (                                  x                  n                                )                                      ⁢                          (                                                x                  n                                -                                  x                                      n                    -                    1                                                              )                                                          S              ⁡                              (                                  x                  n                                )                                      2                                          D      ⁢              (                  x          n                )              =          12.0      xc3x97              (                  0.33          +                      1.02            xc3x97                          (                                                                    a                    ⁢                                          (                                              x                        n                                            )                                                                            b                    ⁢                                          (                                              x                        n                                            )                                                                      +                                                      b                    ⁢                                          (                                              x                        n                                            )                                                                            a                    ⁢                                          (                                              x                        n                                            )                                                                                  )                                      )            
where,
k is the number of division of the ink flow path;
xn is a distance to an nth divided position when the ink flow path is divided into k parts;
S(xn) is a cross section area of the ink flow path in the position of the distance xn from the common liquid chamber;
D(xn) is a cross section coefficient of the ink flow path in the position of the distance xn from the common liquid chamber;
a(xn) is a height of the ink flow path in the position of the distance xn from the common liquid chamber;
b(xn) is a width of the ink flow path in the position of the distance xn from the common liquid chamber; and
xcex7 is an ink viscosity.
In this case, it is preferable that the multiplications and the additions are performed along a path in which a main flow of ink is generated and S(x), S(xn), D(x) and D(xn) are obtained on a cross section perpendicular to the path.
Moreover, it is preferable to perform the multiplications and the additions over the path from the common liquid chamber to the center of the electrothermal conversion element.